Using $sort and $project

Review the following code, which demonstrates how to build the $sort and $project stages of an aggregation pipeline in MongoDB with Java.

$sort and $project

In the following example, we use the Aggregates builder class to build the $sort and $project stages of an aggregation pipeline. This pipeline finds all checking accounts with a balance greater than 1500 and sorts the results in descending order. The query will return only four fields, including a newly computed Euro balance.

The match() method creates a $match pipeline stage that matches incoming documents against the specified query filter and filters out documents that don't match. Then, the sort() method creates a $sort pipeline stage to sort by the specified criteria. Finally, the project() method creates a $project pipeline stage that projects specified document fields. Field projection in aggregation follows the same rules as field projection in queries.

public static void main(String[] args) {
    String connectionString = System.getProperty("mongodb.uri");
    try (MongoClient mongoClient = MongoClients.create(connectionString)) {
        MongoDatabase db = mongoClient.getDatabase("bank");
        MongoCollection<Document> accounts = db.getCollection("accounts");
        matchSortAndProjectStages(accounts);
    }
}

private static void matchSortAndProjectStages(MongoCollection<Document> accounts){
    Bson matchStage =
            Aggregates.match(Filters.and(Filters.gt("balance", 1500), Filters.eq("account_type", "checking")));
    Bson sortStage = Aggregates.sort(Sorts.orderBy(descending("balance")));
    Bson projectStage = Aggregates.project(Projections.fields(Projections.include("account_id", "account_type", "balance"), Projections.computed("euro_balance", new Document("$divide", asList("$balance", 1.20F))), Projections.excludeId()));
    System.out.println("Display aggregation results");
    accounts.aggregate(asList(matchStage,sortStage, projectStage)).forEach(document -> System.out.print(document.toJson()));
}